The present invention relates to a pressure detecting or sensing device, wherein pressure externally applied to the flexible diaphragm of a cell is electrically detected by the movement of a non-compressible filler liquid in the cell.
The known devices for sensing pressure provide an arrangement such that a couple of diaphragms are fixed to opposite cylinder openings to form a cell for containing a non-compressible filler liquid. One of the diaphragms is subjected to atmospheric pressure or vacuum pressure to establish reference pressure, while the other of the diaphragms is exposed to a pressure to be measured, and the pressure differential is measured from the displacement of the diaphragms.
To be more specific, as illustrated in FIG. 1, the pressure sensing cell consists of a hollow cylindrical member 2 made of metal which is sealed by a pair of diaphragms 10 and 10A each disposed over the open ends of the cylindrical member 2. In the cylindrical member 2, insulating filler material 3 is provided having end surfaces 4 and 4A which are spherically concave and to which metal foils 5 and 5A are applied, respectively. The diaphragms 10 and 10A have their peripheral end portions welded to both planer end surfaces of the cylindrical member 2 while the flatness of the diaphragms is maintained. Small liquid chambers each defined by the diaphragm 10 and the side surface 4 and by the diaphragm 10A and the side surface 4A are connected by a conduit 6 which penetrates the insulating material 3. These chambers and the conduit are filled with a non-compressible filler liquid such as silicone oil. The diaphragms 10 and 10A and the metal foils 5 and 5A function as variable capacitors. The metal foils are connected to conduction wires 8 which are extended outside of the device through a bore 9 formed in the hollow cylindrical member 2.
With this structure, however, the prior art devices do exhibit certain drawbacks. Particularly, thermal error is observed.
If the filler liquid is expanded due to the temperature increase, the sensing diaphragms are outwardly expanded and, therefore, the capacitances of the capacitors each defined by the foil 5 and diaphragm 10 and the foil 5A and the diaphragm 10A are varied. Further, if the filler liquid is contracted due to temperature decrease, some vacuum portion, known as Torricelli's vacuum, may be formed in the liquid chambers 12 and 12A if the inward deformation of the diaphragms can not follow the contraction and this degrades the sensing accuracy of the device.